Study: Humans are primary cause of global ocean warming

Pacific and Atlantic Ocean zonal average cross sections (surface to 700 m) of temperature changes for 1955 to 2011. Each globe represents a decadal average. The foreground is the most recent decade and preceding decades are in the background. Red represents warming ocean, white no change, and blue for cooling with respect to a 1957 to 1990 average. Data was provided from the National Oceanographic Data Centre (NODC) World Ocean Database (WOD). Image: Timo Bremer/LLNL

New research by a team of Lawrence Livermore National Laboratory (LLNL)
scientists and international collaborators shows that the observed ocean
warming over the last 50 years is consistent with climate models only if the
models include the impacts of observed increases in greenhouse gas during the
20th century.

Though the new research is not the first study to identify a human
influence on observed ocean warming, it is the first to provide an in-depth
examination of how observational and modeling uncertainties impact the
conclusion that humans are primarily responsible.

"We have taken a closer look at factors that influence these
results," said Peter Gleckler, an LLNL climate scientist and lead author
of the new study that appears in Nature Climate Change. "The bottom
line is that this study substantially strengthens the conclusion that most of
the observed global ocean warming over the past 50 years is attributable to
human activities."

The group looked at the average temperature (or heat content) in
the upper layers of the ocean. The observed global average ocean warming (from
the surface to 700 m) is approximately 0.025 C per decade, or slightly more
than 1/10th of a degree Celsius over 50 years. The sub-surface ocean warming is
noticeably less than the observed Earth surface warming, primarily because of
the relatively slow transfer of ocean surface warming to lower depths.
Nevertheless, because of the ocean's enormous heat capacity, the oceans likely
account for more than 90% of the heat accumulated over the past 50 years as the
Earth has warmed.

In this study the team, including observational experts from the United States, Japan
and Australia,
examined the causes of ocean warming using improved observational estimates.
They also used results from a large multi-model archive of control simulations
(that don't include the effects of humans, but do include natural variability),
which were compared to simulations that included the effects of the observed
increase in greenhouse gases over the 20th century.

"By using a "multi-model ensemble," we were better
able to characterize decadal-scale natural climate variability, which is a
critical aspect of the detection and attribution of a human-caused climate
change signal. What we are trying to do is determine if the observed warming
pattern can be explained by natural variability alone," Gleckler said.
"Although we performed a series of tests to account for the impact of
various uncertainties, we found no evidence that simultaneous warming of the
upper layers of all seven seas can be explained by natural climate variability
alone. Humans have played a dominant role."